九重火山の地殻変動

Abstract

1995年10月に九重火山で生じた水蒸気爆発以後, 我々は繰り返し光波辺長測量とGPS測量を行ってきた。最も大きく変化したSGM・HSS測線(1.1km)では, 約6年間で70cm以上も収縮し, 多くの測線でほぼ直線的な辺長の変化が現在も継続している。茂木モデルの減圧源が硫黄鉱山のあった硫黄山の噴気地帯直下600mの位置に求まった。これらの地殻変動と噴気による放出エネルギーの時間変化との間に相関関係がみられることから, これらの継続的な地殻変動は噴気活動に伴う地熱貯留層の減圧に起因していると考えられる。

A phreatic eruption occurred at Kuju volcano in October 1995.We deployed EDM and GPS networks around the active craters of the volcano just after the eruption.Slope distances of the survey lines in the northern network have tended to contract, whereas those in the southern one tended to extend.The maximum contraction observed in the northern network was 70 cm over 6 years. A spherical volume decrease just beneath the fumarolic area is a plausible model for these changes in slope distances.A noteworthy feature is that over 6 years after the phreatic eruption ended, the deflation rate is still approximately linear.We also estimated the thermal energy discharge by fumaroles in the new craters.Temporal variation of the energy discharge is well correlated with the observed deflation rate.It is strongly suggested that the ground deformation around Iwoyama is caused by the deflation volume of a geothermal reservoir.